Ceramic sheets are of particular importance in the electronics industry for the packaging/mounting of semiconductor integrated devices and other elements, given the high densities attainable in a multilayer ceramic structure. The fabrication of ceramic substrates generally is well-known (see U.S. Pat. Nos. 3,423,517 and 3,723,176) and entails the following well-known processing steps: mixing the ceramic "paint" of ceramic, binder and solvents, casting the "green" sheets, drying the sheets, blanking and punching via holes in the sheets, screening metallurgy into the vias, stacking, laminating, and firing, consisting of driving off the binder and finally sintering the structure. A similar processing profile is followed regardless of the specific components of the ceramic material, alumina, mullite, glass ceramic, etc. Illustration of the processing steps as applied to glass-ceramic structures can be seen in U.S. Pat. Nos. 4,340,436 of Dubetsky et al; 4,234,367 Herron et al, and 4,301,324 of Kumar et al, the teachings of which are herein incorporated by reference.
Of particular concern in processing the ceramic structure is the shrinkage and distortion which the structure undergoes during sintering. In addition to linear, reproducible shrinkage in the X-Y plane, there is non-linear X-Y shrinkage, via bulge, and curvature in the Z direction, which is referred to as camber.
Various methods of reducing the effects of shrinkage and Z-direction distortion have been proposed. Several patented methods, U.S. Pat. No. 4,009,238 of Niedermeier et al, U.S. Pat. No. 3,310,392 of Rhodes and U.S. Pat. No. 3,436,451 of Wasser, teach the use of applied pressure to alleviate camber during sintering and to insure a planar surface. Weights applied to the substrate during sintering have also been used to eliminate camber, along with the corrective measure of applying heat and pressure after sintering to reshape the ceramic (see U.S. Pat. No. 4,340,436 of Dubetsky et al, Col. 4 lines 40-52).
Later teachings have expressed a theory that the Z-direction camber or warpage is the result of the varying rates of shrinkage and different percent volume shrinkage of the ceramic and accompanying metallurgy, in part due to the differing shrinkage onset temperatures and different total volume shrinkage. Differing thermal coefficients of expansion and in situ frictional forces create stress-related non-uniformities. Furthermore, the non-uniform distribution of metallization in the ceramics contributes to the overall distortion. In response to these concerns Brownlow et al (IBM Technical Disclosure Bulletin Vol. 23, No. 5, October 1980, page 1885) and Elderbaum (U.S. Pat. No. 3,879,509) illustrate the use of specially shaped shims, weights, and fusible frames capable of exerting uneven pressure on certain areas of the substrate during lamination and sintering, respectively, in attempts to compensate for and/or equalize the shrinkage. Still others (U.S. Pat. No. 4,109,377, Blazick et al and U.S. Pat. No. 3,978,248, Usami) have attempted to alter the compositions of the ceramic and metallurgy so that the two will be more closely compatible in shrinkage rates and overall volume shrinkage.
Many of the above-cited methods have been useful in lessening the effects of Z-curvature, though not of X-Y shrinkage or distortion. Furthermore, the use of glass-ceramic/copper metallurgy structures occasions new problems. Corrective measures such as the post-sintering heating and pressure reshaping treatment, mentioned above, are ineffective for glassceramic/copper substrates. The glass crystallizes during the sintering step and cannot, therefore, be "remelted" and reformed without extreme difficulty at extreme temperatures at which most metallurgies will melt. A further requirement of a glass-ceramic H/H copper metallurgy substrate is that the binder burn-off and sintering be conducted in a neutral or a reducing atmosphere due to the high oxidizing potential of the copper. As taught in the Dubetsky et al patent, U.S. Pat. No. 4,340,436 an ambient for which the partial pressure of O.sub.2 is controlled, such as H.sub.2 O/H.sub.2, is preferable, since reducing ambients may cause adhesion problems. Necessarily, the substrate surface must be exposed to the ambients in order for the process to be effective. The use of shims, pressure platens or weights on the surface presents a problem in this regard. The Dubetsky solution involves a two-step process whereby the binder burn-off is conducted with the substrate surface unobstructed. The heating process is then halted and an inert weight in the form of a co-extensive platen is applied to the surface of the structure. The structure is then returned to the furnace for completion of the sintering and crystallization. The Dubetsky process is moderately effective in addressing the Z-curvature difficulty; however, it has been determined that a good percentage (33-50%) of the total fired shrinkage and accompanying distortion, camber and via bulge, occurs during the binder burn-off step when the top surface of the substrate must have free, unobstructed access to the atmosphere. During the burn-off step, very small temperature gradients have a pronounced effect on the amount of shrinkage and distortion. Furthermore, neither Dubetsky nor any of the other prior methods are able to reduce the problems of X-Y shrinkage and X-Y distortion. The Elderbaum patent, in fact, teaches away from preventing lateral shrinkage, (see Column 2, lines 14-16), lest the sheet rupture. In general, the industry has accepted the inevitability of shrinkage and attempted to design around the shrinkage problem. Provided that the shrinkage is uniform or "reproducible", the substrate can be sized to allow for the anticipated shrinkage either before or after sintering. Another U.S. patent issued to Dubetsky, U.S. Pat. No. 4,259,061, teaches a method of firing the ceramic element on a refractory material setter tile to assure uniform shrinkage. Again, however, the shrinkage presumably cannot be eliminated.
It is therefore one of the objects of this invention to provide a means and method for fabricating ceramic sheets which eliminates shrinkage, camber and distortion in said sheets.
It is a further object to provide a method for preventing shrinkage, camber and distortion of glassceramic sheets while allowing atmospheric access to the surface of the substrate during firing.
It is another object of the invention to provide planar ceramic substrates requiring a continual uninterrupted burn-off and sintering cycle.
It is still another object of the invention to provide for planarity and dimensional integrity in a ceramic substrate or in a multilayered metallized ceramic substrate (i.e. MLC), without need of wasteful resizing or reworking.